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To answer your question, yes, the 80 amp and 100 amp Denso alternators use the same regulator, so you should be good to go swapping in a Denso regulator from a Denso 80 amp alt from a 700. However, I thought most 700s were almost all Bosch, leastwise up to 1989 or so, also that all 240s were Bosch. Totally different regulators.
The original Denso regulator p/n's were 126000-1130 and -1320 according to the Volvo Pocket Data service book. Newer p/n's such as -1322 should be equivalent. BTW double-check your Denso p/n. The Volvo 80 and 100 amp Densos are listed with p/n's starting 100211- not 101211-, and I don't see the 101211- listed anywhere for Volvo cars. Suffices -8620 and -8630 are 100 amps alts, while -8370 is listed as 80 amp. Newer p/n's with the same first two digits may well be equivalent.
First question I've got to ask is how old is the battery? Anything over 3 years puts it high on the main suspect list, not the alternator. Anything over 5 years and you're on borrowed time. If you get over 7 years, well that one was one very good battery and the odds are you'll never be able to buy a battery as good as that one now. An auto repair shop selling batteries will have a load tester and will often do it for free. For load testing, take the battery in with a good partial charge, like 70%-80% charge. A weak battery with a fresh 100% charge may pass a load test and a good battery nearly discharged may fail a load test.
Second question is did you do a voltage test for regulator output? You need to measure at the back of the alternator between B+ and D-, not at the battery. At 1,900+ rpm with all loads off, you should be seeing no less than 13.85V (perhaps with DRL and electric fan on), and should be seeing closer to 14.2V (14.15V to 14.35V is a good range), and no more than 14.8V which is now starting to overcharge. The Volvo spec for Denso regulators is 13.8V-14.6V with a battery at 75% charge and 14.1V-14.9V at full charge. That's with an ambient temp of 25 deg C (75 F), so a bit lower on cold days is okay.
Did you do voltage drop tests to verify cabling and connections? With a partial load on and at higher than idle, between B+ and the battery positive post, you should be seeing no more than a 1/8V-1/4V (0.2V) voltage drop, otherwise there's a cabling problem. Also, no more than 0.2V between the battery negative post and D-, indicating a poor ground connection, a much more common problem.
And further to your other comments, an 80 amp alternator out of a 700 should be quite adequate in a '93 940 NA except under the most demanding of conditions, such as short trips, in creeping rush hour with all loads on high in the middle of winter with an older battery that's no longer holding a full charge. An 80 amp alt out of a 240 uses a different mounting bracket as I recall. Both 80 and 100 amp alternators were used in 700s and 900s, mostly Denso 100 amp in the later 900 years. In 700s, I recall they only gave you a 100 amp alt in the later GLE and GLT models, which came with power sunroof and power driver seat, sometimes also a stereo pre-amp. Even then, they would only be needed if you tended to use those frequently. In all other regards, the electrical loads and accessories are the same between the 940s and later 740s. Similarly with 240s compared to 740s, the 80 amp alt became standard issue when extra loads like electric radiator fans and power window/door locks becoming standard. Volvos had electric defrost windows and seat heaters since the 140 days using a 65 amp alt. Whereas only the higher trim levels seemed to get the 100 amps in 700s, in 940 production the trim levels seemed to vary between getting things like power sunroof and power driver seat in the NA and T models, so perhaps that's why the 100 amp became the default. I've also seem assorted 940 NA and T with and without alloys and wagon roof rails. I've always figured as they got close to the end of production they just used up whatever chassis were coming down theline and what was overstock on the shelf for options.
In reference to parasitic draws, although possible, it's not at all common for the alt itself to become a parasitic draw. Mostly it's losing a diode phase and producing less output. I've been down the battery discharge and parasitic load road many times what with driving less and going through periods where one of our 940s isn't driven for a few weeks then only lasting a few days, also batteries reaching their best before date. Things like the clock, ECU and radio are normal parasitic draws that a good battery can cope with for many weeks, even months, but with a weaker battery it may only be able to hold a charge for a matter of days. Add in a lot of short driving and the battery never gets a chance to properly recharge. Put a charger on the battery, as you likely did, and you can be good for another few weeks of normal starting until finally getting down to an overnight no-start. Swapping in your spare alt and having things back to normal would then be a coincidence.
The only time I've had a mystery parasitic draw issue with a half-decent battery was a CD disc changer in the trunk that would occasionally sit there forever silently still trying to establish tracking of a mounted CD-R (wouldn't mount a CD-RW). Pulling the fuse confirmed my suspicions that it was the culprit, soon ripped out and now on the shelf. Between having MP3, USB and Bluetooth in a newer radio, I no longer have a need for CD, let alone cassette.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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